FIELD OF THE INVENTION
The invention relates to a method and an apparatus for metering a reagent into a flowing medium, for instance for metering ammonia into a catalytic conversion of nitrogen oxides, contained in an exhaust or flue gas, using ammonia.
In the catalytic conversion of at least two reagents of a flow medium, if high degrees of settling and at the same time negligible leakage of the reagents are to be attained, then it is necessary to furnish the reagents to a catalytic converter in a suitable stoichiometric ratio to one another. In the case of the catalytic conversion of nitrogen oxides contained in an exhaust gas or flue gas by the method of selective catalytic reduction (SCR) with ammonia as the reducing agent, for instance, it is necessary for the nitrogen oxides and ammonia to be present in approximately equal portions, averaged over time, at the catalytic converter.
Adjusting the requisite stoichiometric ratio of nitrogen oxides to ammonia, or to some substance such as urea that can be converted into ammonia, can be performed with satisfaction only if the nitrogen oxide concentration in the exhaust or flue gas can be measured, or can be determined comparatively accurately through a performance graph diagnosis.
However, measuring the nitrogen oxide concentration requires comparatively major effort and entails comparatively great expense. For instance, such measurements can therefore be carried out only in large SCR systems in power plants, but once again a local leakage of ammonia caused by nitrogen oxide skewing can only be unsatisfactorily prevented. Yet avoiding leakage of ammonia must be sought under all circumstances, since ammonia is poisonous and in even extremely slight concentrations ammonia causes annoyance to the human being from its smell (odor threshold approximately 5 ppm).
Since the nitrogen oxide concentration in fossil-fueled power plant flue gases also only varies slowly, because of load changes that are only slowly completed over time, in power plants ammonia metering is carried out with the aid of measurements of nitrogen oxide and nitrogen oxide leakage, with relatively long time constants. Ammonia metering for flue gases with only slowly completed changes in nitrogen oxide concentration is thus achieved somewhat satisfactorily.
Removing nitrogen from flue gases emitted by Diesel engines and lean-running engines is significantly more difficult. Due to different operating states and rapid load changes in such engines, a system constructed for such engines, with a controlled Diesel catalytic converter (CDC), which likewise works by the SCR process with respect to nitrogen oxide reduction, must be constructed for the resultant major fluctuations in the volumetric flow of exhaust gas, exhaust gas temperature, and nitrogen oxide concentration in the exhaust gas. Since ammonia itself is hazardous and therefore cannot be carried in vehicles, such as passenger cars, trucks, buses, locomotives and ships, the requisite reducing agent is instead carried in the vehicle in the form of an aqueous urea solution, for instance, from which ammonia is then generated, ideally in precisely the quantity needed at the moment.
Since the use of currently known nitrogen oxide sensors in vehicles is extremely improbable because of the major effort and expense involved, the attempt has been made to ascertain the nitrogen oxides produced by the engine per unit of time by a performance graph comparison, adding metered quantities of a suitable reducing agent to these quantities of nitrogen oxide, thus called up from the graph or calculated, and simulating the settling capacity of the catalytic converter in an on-board diagnosis system (see German Published, Non-Prosecuted Application DE 43 15 278 A1, for instance). In a corresponding embodiment of the catalytic converter, in other words with a suitable catalyst adsorption and desorption characteristic, for instance as defined by German Published, Non-Prosecuted Application DE 43 09 891 A1, corresponding to co-pending U.S. application Ser. No. 08/218,358, filed Mar. 25, 1994, the adding of ammonia in metered fashion to the exhaust gases of Diesel and lean-running engines is solved to some satisfaction by means of changing the nitrogen oxide concentration in a way that is completed quickly.
However, it has been definitively found that in the metered addition of the reducing agent in an SCR or CDC system, there is thus far no suitable control capability, and as a result if there is a defect, for instance in the catalytic converter or at the metering valve, annoyance can be created by excessive leakage of ammonia.